Iterating multiple Outlook COM Object fails - powershell

Question

We're having a very strange problem. When iterating through multiple elements in an array of public folder names, PowerShell sometimes throws an error. But not always.
When running the code below with as input only one element it works fine but when multiple elements are defined the second iteration throws an error.
According to this Microsoft article one should Release the COM-Object, but this doesn't work either.
Code
Param (
[String]$Mail = 'User#donain.com',
[String]$ImportFile = 'C:\Scripts\Import.txt'
)
$Import = Get-Content $ImportFile
$Start = "\\Public Folders - $Mail"
Add-Type -AssemblyName 'Microsoft.Office.Interop.Outlook'
foreach ($L in $Import) {
$PSTFile = "$ExportFolder\$($L -replace '[^A-Za-z0-9-_ \.\[\]]', ' ').pst"
$Outlook = New-Object -ComObject Outlook.Application -Verbose:$false
$Namespace = $Outlook.GetNameSpace('MAPI')
$AllPublicFolders = $Namespace.Folders | where FolderPath -EQ $Start | ForEach-Object {
$Start = $Start + '\All Public Folders'
$_.Folders | where FolderPath -EQ $Start
}
$Split = $L.Split('\')
$Folder = Switch ($Split.Count) {
1 {$AllPublicFolders.Folders.Item($Split[0])}
2 {$AllPublicFolders.Folders.Item($Split[0]).Folders.Item($Split[1])}
3 {$AllPublicFolders.Folders.Item($Split[0]).Folders.Item($Split[1]).Folders.Item($Split[2])}
4 {$AllPublicFolders.Folders.Item($Split[0]).Folders.Item($Split[1]).Folders.Item($Split[2]).Folders.Item($Split[3])}
5 {$AllPublicFolders.Folders.Item($Split[0]).Folders.Item($Split[1]).Folders.Item($Split[2]).Folders.Item($Split[3]).Folders.Item($Split[4])}
6 {$AllPublicFolders.Folders.Item($Split[0]).Folders.Item($Split[1]).Folders.Item($Split[2]).Folders.Item($Split[3]).Folders.Item($Split[4]).Folders.Item($Split[5])}
7 {$AllPublicFolders.Folders.Item($Split[0]).Folders.Item($Split[1]).Folders.Item($Split[2]).Folders.Item($Split[3]).Folders.Item($Split[4]).Folders.Item($Split[5]).Folders.Item($Split[6])}
8 {$AllPublicFolders.Folders.Item($Split[0]).Folders.Item($Split[1]).Folders.Item($Split[2]).Folders.Item($Split[3]).Folders.Item($Split[4]).Folders.Item($Split[5]).Folders.Item($Split[6]).Folders.Item($Split[7])}
9 {$AllPublicFolders.Folders.Item($Split[0]).Folders.Item($Split[1]).Folders.Item($Split[2]).Folders.Item($Split[3]).Folders.Item($Split[4]).Folders.Item($Split[5]).Folders.Item($Split[6]).Folders.Item($Split[7]).Folders.Item($Split[8])}
}
Write-Verbose "Folder '$($Folder.FolderPath.TrimStart($Start))'"
Write-Verbose "Add PST"
$NameSpace.AddStore($PSTFile)
$PSTStore = $NameSpace.Stores | where {$_.FilePath -eq $PSTFile}
Write-Verbose "Copy content to PST"
$Folder.CopyTo($PSTStore) | Out-Null
Write-Verbose "Remove PST"
$PST = $NameSpace.Stores | where {$_.FilePath -eq $PSTFile}
$PSTRoot= $PST.GetRootFolder()
$PSTFolder= $NameSpace.Folders.Item($PSTRoot.Name)
$NameSpace.GetType().InvokeMember('RemoveStore',[System.Reflection.BindingFlags]::InvokeMethod,$null,$Namespace,($PSTFolder))
$Outlook.Quit()
[System.Runtime.Interopservices.Marshal]::ReleaseComObject($Outlook)
Remove-Variable Outlook
Start-Sleep -Seconds 5
}
ImportFile
WEUR - COMPANY\DAF\Tableau de bord IB\Année 2002\07 juillet
WEUR - COMPANY\DAF\Tableau de bord IB\Année 2002\08 Août
Error
Failed for path 'WEUR - DOMAIM\DAF\Tableau de bord IB\Année 2002\08 Août': You cannot call a method on a null-va
lued expression.

Answer 1

Found the problem:
$AllPublicFolders = $Namespace.Folders | where FolderPath -EQ "\\Public Folders - $Mail" | ForEach-Object {
$_.Folders | where FolderPath -EQ "\\Public Folders - $Mail\All Public Folders"
}